1. Field of the Invention
The invention relates to a white light emitting diode, and especially relates to a polarized white light emitting diode.
2. Description of the Prior Art
A white light emitting diode (LED) is formed by plural color light emitting diode chips. Another white light emitting diode (LED) is mostly formed by multiple color phosphor powders and one of the blue light emitting diode chip and the ultraviolet light emitting diode chip.
Though the first white LED formed by plural color light emitting diode chips has the capability for the adjustable color light, the different materials of plural color light emitting diode chips could make the color temperature rise up following the varied color and the hard mixing of the color light and etc. The disadvantages of the white LED formed by multiple color phosphor powders and the ultraviolet light emitting diode chip are the insufficient illumination and causing damage due to the ultraviolet light. Therefore, said white LED must be to improve the emitting efficiency thereof and to add an optical unit thereon for avoid the ultraviolet light emitting outward.
The white LED is predominantly formed by the blue light emitting diode chip and the yellow phosphor powder, wherein the white light is emitted by mixing the blue light with the yellow light generated by the yellow phosphor powder excited by the blue light. However, the intensity of the blue light is larger than the intensity of the yellow light in the mixing process of said white LED, so the disadvantage of said white LED are high color temperature and glare caused, so the color rendering is unable to rise to 80 and the illumination and the color are not uniform to produce yellow circle phenomenon. Otherwise, the wave of the blue light emitted from the blue LED increases following the operation temperature, so the color of the white light by mixing the blue light and the yellow light is varied.
However, the white light, which is emitted by said white LEDs, is non-polarized. In illumination application, the non-polarized light causes glare so user feels uncomfortable as reading under the non-polarized light. How to make the white light emitted from the white LED polarize, own high extinction ration and decrease glare is an urgent problem in the field.
Moreover, the current liquid crystal display (LCD) is mainly composed of a liquid crystal layer, a substrate, a back-lighting device, two polarizing sheets and a filter. The non-polarized white light emitted by the white LED is used as the light source of the back-lighting device and is polarized by polarizing sheets, so the polarized white light produces the spatial light modulation as penetrating through the liquid crystal layer and the LCD could work. The polarized white light directly emitted from the white LED can increase the illumination and reduce the usage of the polarizing sheets so as to decrease cost.
For making the white light emitted from the white LED polarize, there are four prior arts described as below. The prior art (U.S. Pat. No. 7,091,661) teaches a LED chip, a phosphor powder and a reflective polarizer. The phosphor powder is placed above the LED chip and between two polymer filters. The reflective polarizer is placed above the phosphor powder. A first light emitted from the white LED enters the phosphor powder through the polymer filter, and a second light is excited. The light formed by mixing the first light and the second light is emitted from another polymer filter and goes through the reflective polarizer, so a polarized light is generated. Said reflective polarizer is a polarizing multi-sheets or an absorptive polarizing sheet. Another prior art (U.S. Pat. No. 7,766,495) teaches a reflective cover placed above a LED chip, so the light emitted from the LED chip is reflected by the reflective cover to one direction and a polarized light is outputted.
Another prior art (U.S. Pat. No. 7,339,736) teaches a reflective polarized multi-sheets made from polymer and placed above a non-polarized light. A polarized is formed by the non-polarized light going through the reflective polarized multi-sheets, so can be used for the back-lighting device of the LCD to increase the illumination. However, the reflective polarized multi-sheets made from polymer is easy exhaustion in the operation due to the high operation temperature and the exposure under the ultraviolet light. Thus, how to make the reflective polarized multi-sheets durable is another urgent problem in the field.
Another prior art (U.S. Pat. No. 7,614,757) teaches a reflective metallic wire-grid polarizer used for the back-lighting device of the LCD to increase the illumination. The reflective metallic wire-grid polarizer is made of plural nanometer metallic wire array placing on the a transparent substrate, and the advantages thereof are wide range of the operation wave, high extinction ratio, large light radiation angle and great durable level. However, the nanometer structure of the reflective metallic wire-grid polarizer is easy-damaged, so how to protect the reflective metallic wire-grid polarizer is another urgent problem in the field.